Method for the production of killed,unalloyed or low-alloy,aluminum containing steel with low carbon content

ABSTRACT

THE INVENTION RELATES TO A METHOD FOR THE PRODUCTION OF KILLED, UNALLOYED OR LOW-ALLOY, ALUMINUM CONTAINING STEEL HAVING A LOW CARBON CONTENT, ACCORDING TO THE CONTINUOUS CASTING METHOD, WHEREIN ALUMINUM IS ADDED TO THE LIQUID METAL IN TWO PORTIONS, THE FIRST PORTION BEING ADDED AT A PERIOD BEFORE THE LIQUID STEEL IS CAST INTO THE TUNDISH AND BEING OF A QUANTITY SUFFICIENT TO BIND THE MAJOR AMOUNT OF OXYGEN PRESENT IN THE LIQUID STEEL, WITH THE PROVISION THAT THE CASTING STREAM COMING INTO CONTACT WITH AIR CONTAINS ONLY BETWEEN 0.005 AND 0.01% OF METALLIC ALUMINUM, AND THE SECOND PORTION BEING ADDED AT A STAGE WHEN THE CASTING STREAM WILL NO LONGER GET INTO CONTACT WITH THE ATMOSPHERE AND BEING OF A QUANTITY SUFFICIENT TO OBTAIN THE DESIRED ANALYSIS. ACCORDING TO THE INVENTION A MATERIAL IS OBTAINED WHOSE SURFACE IS LARGELY FREE FROM ALUMINUM OXIDES AND THUS IS SUITABLE FOR DEEP-DRAWING PURPOSES.

United States Patent 3,730,704 METHOD FOR THE PRODUCTION OF KILLED, UNALLOYED OR LOW-ALLOY, ALUMINUM CONTAINING STEEL WITH LOW CARBON CONTENT Thorwald Fastner and Gerhard Papp, Linz, Austria, assignors to Vereinigte Osterreichische Eisenund Stahlwerke Aktiengesellschaft, Linz, Austria N0 Drawing. Filed Jan. 26, 1970, Ser. No. 5,912 Claims priority, application Austria, Jan. 28, 1969, A 826/69 Int. Cl. C22c 39/02 US. Cl. 75-129 3 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a method for the production of killed, unalloyed or low-alloy, aluminum containing steel having a low carbon content, according to the continuous casting method, wherein aluminum is added to the liquid metal in two portions, the first portion being added at a period before the liquid steel is cast into the tundish and being of a quantity sufficient to bind the major amount of oxygen present in the liquid steel, with the provision that the casting stream coming into contact with air contains only between 0.005 and 0.01% of metallic aluminum, and the second portion being added at a stage when the casting stream will no longer get into contact with the atmosphere and being of a quantity sufiicient to obtain the desired analysis. According to the invention a material is obtained whose surface is largely free from aluminum oxides and thus is suitable for deep-drawing purposes.

The present invention relates to a method for the production of killed, unalloyed or low-alloy, aluminum containing steel having a low carbon content, according to the continuous casting method under addition of aluminum as deoxidizing agent, the liquid steel being cast under contact with the oxygen containing atmosphere into a tundish, and from said tundish into a cooled mould whereupon the solidifying bar is drawn and subjected to further cooling.

Killed, aluminum containing steels with low carbon content are primarily processed to cold rolled sheets capable of being deep-drawn and meeting the highest demands regarding surface quality. Such sheets are destined primarily for the manufacture of car bodies.

For the production of sheets having a quality that permits of deep-drawing or use for the manufacture of car bodies a cast processing material is necessary which is deoxidized by means of aluminum and whose surface zone is very clean, i.e. largely free from non-metallic inclusions, especially from macroscopic inclusions, which otherwise would get to the surface of the rolling stock during rolling and would lead to the formation of linear flaws, so-called bands, in the surface of the sheets.

When conventional casting methods are employed either to cast ingots or for continuous casting, the obtained product representing the material to be processed in the rolling mill, does not meet the requirements mentioned. The marginal zone of the cast product deoxidized' with aluminum is greatly impurified by non-metallic inclusions, especially macroscopic ones.

In steel deoxidized with aluminum these macroscopic inclusions are primarily aluminum oxides. In order to remove the surface flaws occurring in bands on the sheets and which are caused by the inclusions, the surface zone of the processing material has to be removed by flame scarfing. The layer to be removed is particularly thick with material destined for the manufacture of car bodies 3,730,704 Patented May 1, 1973 or deep-drawing purposes because the surface of the finished product has to meet highest demands; therefore the entire layer close to the surface and containing inclusions has to be removed. Flame scarfing is rather expensive and results in a loss of material of between about 5 and 15%.

The invention is aimed at avoiding these disadvantages and difficulties by attempting to prevent the formation of a layer rich in inclusions. It has been found that in continuous casting there is a connection between the casting speed, which in general is lower than in conventional casting of ingots (rapid casting), and the frequency of surface flaws occurring as bands on the sheet: a reduction of the casting speed leads to an increase in the number of bands in the surface. The aluminum oxides are formed under the influence of the atmospheric oxygen on the dissolved aluminum in the casting stream and unite to macroscopic agglomerations which are deposited primarily in the marginal zone of the slabs when it is no longer possible to eliminate them.

A number of methods have become known for reducing the content of non-metallic inclusions in the steel, including degassing in vacuum, flushing the steel melt with inert gas, or the use of special deoxidizing agents. These measures are, however, employed prior to casting the steel melt. Whereas they diminish the frequency of inclusions contained in the steel, they are not capable of preventing the formation af aluminum oxides in the casting stream on account of the influence of air.

On its path from the metallurgical refining vessel to the mould the liquid steel has to pass an oxidizing atmosphere at least twice.

During the first passage, i.e. during tapping of the melt from the refining vessel, no damaging oxidation takes place; in that stage one portion of the aluminum is pre-charged into the ladle, the other portion is added to the tapping stream.

The second passage of the liquid steel through the oxidizing atmosphere takes place at the continuous casting plant when the ladle is emptied, in correspondence with the lowering speed of the bar, into the tundish serving as a butter. It is here that the undesired oxidation of the aluminum in the casting stream occurs. Owing to the fact that the pouring rate employed in continuous casting is very low and fixed within a very narrow range on account of the technological conditions it is not possible to eliminate the influence of the oxidizing atmosphere on the casting stream by increasing the casting speed. It has been proposed to pour the steel from the ladle under a protective gas atmosphere. A realization of such proposals is obviated, however, by constructional difliculties and the incurrence of high costs so that until now such method has not been employed on large scale in steel works.

If no special measures are employed when the steel runs from the tundish into the mould, as e.g. a casting tube ending below the casting level in the mould, the steel may come into contact with the oxidizing atmosphere a third time.

The method according to the invention solves the given problems in that the aluminum is added in two portions, the first portion being added at a stage before the liquid steel is poured into the tundish and being of a quantity sufficient to bind most of the oxygen present in the liquid steel, with the provision that the casting stream coming into contact with air contains only between about 0.005 and 0.01% of metallic aluminum, and the second portion being added only when the casting stream will no longer get into contact with the atmosphere and being of a quantity suflicient to obtain the desired analysis.

Owing to the addition of aluminum in two portions as provided by the method according to the invention the described disadvantageous influence of the oxidizing atmosphere on the casting stream leaving the ladle or the tundish is eliminated in a most simple manner and a cast product is obtained having a level aluminum content over the entire cross-section.

The first portion of aluminum may be added at any moment before the steel is poured into the tundish, so e.g. while the hot metal is tapped from the refining vessel or while the steel is in the ladle. The second portion of aluminum may be added continuously into the mould.

When using a tundish with a casting tube ending below the casting level in the mould the second portion may be added into the tundish.

Owing to the addition of aluminum as provided by the invention the casting stream being in contact with the atmosphere contains only a negligible amount of metallic aluminum. Thus it is prevented that deteriorative alumina particles form in the casting stream under the influence of the oxidizing atmosphere.

The method according to the invention is illustrated by the following examples:

EXAMPLE 1 In a 50 mt. converter pig iron of the following composition: 4.16% C., 0.73% Si, 1.11% Mm, 0.126% P, 0.036% S, remainder substantially iron was converted to steel by top-blowing oxygen. The amount of steel produced was 50.4 mt. The tapping temperature was 1610 C. The steel contained 0.071% of dissolved oxygen. The first aluminum portion of 51 kg. was added into the ladle in the form of billets. The steel was poured from the ladle into the tundish. The steel temperature in the tundish was 1540 C. During pouring a sample of the steel was taken and its aluminum content was determined to amount to 0.007%. The sample contained 0.014% of oxygen. 36 kg. of aluminum was added as second portion into the tundish in the form of an aluminum wire of 8 mm. diameter; this wire was inserted through a guiding tube made of steel. The slantingly arranged guiding tube ended slightly short of the steel bath surface in the vicinity of the area of impingement of the pouring steel. The aluminum wire was fed into the tundish at a rate of 5.64 m./rnin. The cast finished product had the following analysis: 0.05% C., 0.28% Mn, 0.017% P, 0.022% S, 0.056% of metallic aluminum dissolved in the steel, lbalance iron; the measures employed for casting were the following:

Withdrawal speed of the bar: 0.58 m./min.

Casting speed: 1.07 mt./min.

Casting period: 47 min.

Cross section of the bar: 1050 x 225 mm.

The slabs were flame scarfed with 2.5% loss of material. When the cold rolled panels produced from these slabs were sorted, the waste owing to linear surface flaws amounted only to 014%.

With cold rolled sheets produced from melts under largely the same conditions with the exception that the aluminum was added in customary manner exclusively into the ladle and with flame scarfing entailing a loss of material, it was shown after sorting of the panels that the waste owing to surface flaws arranged in bands amounted to 4.8%.

EXAMPLE 2 In a 50 mt. converter pig iron of the following composition: 3.97% C, 0.81% Si, 1.21% Mn, 0.103% P. 0.030%

S, remainder substantially iron was converted to steel by top blowing oxygen. 49.7 mt. of steel were produced. The tapping temperature was 1620 C. The steel contained 0.082% of dissolved oxygen. 66 kg. of aluminum was added in the form of slabs into the ladle as first portion. The steel was poured from the ladle into a tundish. The steel temperature in the tundish was 1545 C. During teeming the ladle into the tundish a sample of the steel was taken and its aluminum content was determined to be 0.012%. The sample contained 0.009% of oxygen. The second portion of aluminum, namely 33 kg., in the form of an aluminum wire having an 8 mm. diameter, was added as described in Example 1 into the tundish with a feeding speed of 6.25 m./min. The cast finished product had the following analysis: 0.04% C, 0.34% Mn, 0.015% P, 0.02% S, 0.062% of metallic aluminum, dissolved in the steel, remainder substantially iron; the measures employed for casting were the following:

Withdrawal speed of the bar: 0.60 m./min. Casting speed: 1.27 mt./ min.

Casting period: 39 min.

Cross section of the bar: 1200 x 225 mm.

The slabs were flame scarfed with a 2.5% loss of material. When the cold rolled panels produced from these slabs Were sorted, the waste owing to surface flaws arranged in bands amounted only to 1.2%

With cold rolled sheets produced from steel melts under largely the same conditions with the exception that the aluminum was added in customary manner exclusively into the ladle and with flame scarfing entailing a 4% loss of material, it was shown after sorting of the panels that the waste owing to surface flaws arranged in bands amounted to 3.8%.

What we claim is:

1. A method for the production of aluminum-containing killed steel for continuous casting, wherein the liquid steel is conveyed as a first casting stream which is in contact with the atmosphere from the ladle into a tundish and thereafter as a second casting stream from the tundish into a mold, comprising adding a first predetermined portion of aluminum to the liquid steel before the steel reaches the tundish to substantially kill the steel, the quantity of said added aluminum being sufiicient to give a metallic aluminum content substantially homogeneously distributed throughout the steei in the tundish of about 0.005% to 0.010% by weight of the steel adding a second predetermined portion of aluminum to the liquid steel at a stage when the second casting stream will no longer contact the atmosphere to alloy the steel and to obtain a final aluminum content of at least 0.20% by weight of the steel, and thereafter continuously casting the steel to produce a casting with improved surface characteristics.

2. The method set forth in claim 1, wherein the second portion of aluminum is added continuously into the mold.

3. The method set forth in claim 1 wherein a tundish is employed comprising a casting tube extending into said mould and terminating below casting level and said second portion of aluminum is continuously added into said tundish.

References Cited UNITED STATES PATENTS 3,459,537 8/1969 Hornak -129 X 3,189,956 6/1965 Longden 75-60 X CHARLES N. LOVELL, Primary Examiner J. E. LEGRU, Assistant Examiner 

